The use of hydrogen (H.sub.2) as fuel for burners of all kinds, for example for combustors in combustion chambers of gas turbines, has the advantage of an especially high reactivity and thus an extraordinary large stability in the combustion. This stable combustion is achieved even if there is an excess air supply as is the case in the combustion chambers of gas turbines.
Publications relating to combustion techniques by Heywood and Mikus show that a reduction in the formation of nitrogen oxides (NO.sub.x) can be achieved in combustion flames with a sufficiently high air excess if the mixing quality of air and fuel is increased. According to Heywood and Mikus, the NO.sub.x formation can be minimized by a completely homogeneous fuel-air mixture as can be attained, for example, by premixing of the fuel and air upstream of the combustion flame proper as viewed in the gas flow direction. A respective suggestion of a homogeneous premixing of the fuel and air supply with hydrogen as fuel, has been made by Pratt and Whitney of Canada. In spite of the advantages that are attained by the premixing with regard to the reduction of nitrogen oxides emissions in engine exhaust gases, there is a substantial drawback in such premixing in that flame flashbacks from the combustion chamber back into the premixing area can happen. Such flame flashbacks are very dangerous.
U.S. Pat. No. 4,100,733 (Striebel et al.) discloses a premix combustor with elaborate efforts to reduce "noxious contaminants" from engine exhaust gases. More specifically, a stable operation without flame flashbacks and the reduction of NO.sub.x are the goals of Striebel et al. This aim is achieved according to Striebel et al. by a plurality of primary tubes wherein fuel and air are premixed at low fuel flow rates and a plurality of secondary tubes for further mixing once a threshold fuel flow rate has been reached. Such stepwise premixing achieves a reasonably homogeneous fuel air mixture prior to entry into the combustion chamber and presumably flashbacks are avoided as long as low BTU fuels are used as is emphasized by Striebel et al. A substantial risk of flashbacks, however, cannot be avoided by the teachings of Striebel et al. if the fuel is hydrogen having very large flame velocities.
The above discussed first group of conventional burners or combustors which uses premixing of hydrogen and air generally requires burners of relatively simple construction. For example, a hydrogen distribution chamber having a plate configuration is inserted into the combustion chamber, whereby the hydrogen flows in a direction crosswise to an air flow direction. The air flow direction is referred to herein as the main or primary flow direction, while the hydrogen flow direction is referred to as the secondary flow direction. The hydrogen distribution chamber includes a multitude of air guide tubes extending in the main flow direction as shown by Striebel et al. Each tube has an inlet and an outlet opening for the air. Each air guide tube communicates through small bores or holes with the hydrogen distribution chamber. These bores or holes are positioned close to the inlet opening of the respective tube so that premixing can take place in each tube. If hydrogen is introduced into the hydrogen distribution chamber, it flows in the secondary flow direction crosswise to the primary flow direction toward the individual bores or holes in the tubes and thus can enter into the air guide tubes which function as premixing tubes. As air is passed through these air guide tubes hydrogen and air are mixed with each other within the air guide tubes before entry of the air fuel mixture into the combustion chamber. Such an arrangement of the hydrogen distribution chamber provides a substantially simplified structural configuration of the burner because individual ducts for the hydrogen to the individual air guide tubes or to the individual combustion zones are not needed.
A second group of hydrogen combustors that works without remixing of air and hydrogen recognizes the importance of the mixing degree for reducing the generation of NO.sub.x in the combustion of hydrogen. This second group of combustors uses diffusion combustion for which an increased number of hydrogen injection nozzles are required. Such nozzles are normally conventional vortex twist generating nozzles. Reference is made in this connection to TRUD by Kusnetzov, published in Russia, and to publications by Motoren-Und Turbinen-Union (MTU) of Munich, Germany. The Kusnetzov principle published in TRUD for example permits increasing the total number of combustion flames over the available burner surface area by a factor of 5 or larger compared to other conventional hydrogen burners. Thus, a combustion chamber conventionally with a given number of combustion flames, for example 30 such flames, can be modified to have 150 or more flames over the entire available burner surface area facing into the combustion chamber. Each of these individual combustion flames still has a diameter of about 20 mm. The TRUD or Kusnetzov system has its limitations in further increasing the number of hydrogen injection nozzles, because the increased number of combustion zones also requires increasing the number of individual hydrogen supply pipelines.
U.S. Pat. No. 3,504,994 (Desty et al.) and U.S. Pat. No. 3,870,459 (Desty et al.) disclose fluid fuel burners falling into the second group of burners using diffusion mixing. The air is supplied through a plurality of tubes which offer a low resistance to air flow making the Desty et al. system particularly suitable for use with natural draught. The fluid fuel is supplied through the gaps between the air supply tubes or through a layer of metal sponge positioned in the gaps between the tubes. Temperature variations cause expansions and contractions of the air tubes, whereby the flow cross-sectional dimensions of the gaps between the air tubes are not dimensionally stable. Hence, the fuel supply is not stable either.
There is room for improvement, especially with regard to the reduction of NO.sub.x in diffusion burners. The disclosure of U.S. Pat. No. 3,504,994 (Desty et al.) tries to improve the fuel air mixing by a baffle plate that has holes surrounding the outlet ends of the air supply tubes, whereby fuel flow ring gaps are formed that surround the air outlet ends of the tubes directly below the baffle plate. While the baffle plate may improve the mixing it will not necessarily improve the steadiness of the fuel supply. Similar considerations apply to an end plate with fuel exit holes which direct the fuel jets in parallel to the air jets, thereby neither improving the mixing nor the NO.sub.x reduction.